High temperature alloy



HIGH TEMPERATURE ALLOY James D. Nishet, Pittsburgh, Pa., assignor toGeneral Electric Company, a corporation of New York No Drawing.Application December 24, 1953, Serial No. 400,379

6 Claims. (Cl. 75134) This invention relates to alloys which have highstrength at elevated temperatures, particularly at about 1500 F., andwrought structural articles made from such an alloy.

The'devel'opment'of the gas turbine and similar devices has created ademand for materials which have high mechanical properties as well asdimensional and physicochemical stability under high stress attemperatures up to 1500 F. Because of the uses to which these alloys arefrequently put, it is essential that "they be capable of withstandingthese high stresses at high temperatures for long periods of time.

Many of the known alloys which "have acceptable mechanical properties atelevated temperatures require casting techniques and canbe forged,rolled, or machined only with great difiiculty. Therefore, articleswhich must be manufactured within close dimensionaltolerancesfrom thesealloys are restricted by practical considerations to expensive precisioncasting methods. It is, therefore, obvious that a wrought alloy havingthe required'prop'er tie's would permit more economical massproductionmethods to be used and result in a better and cheaper :product.

All of the previously known wrought alloys (i. e., alloys which permiteconomical, conventional forming operations such as forging, drawing,etc.) which also have acceptable high temperature properties,incorporate either high percentages of cobalt, nickel, molybdenum andnited States Patent tungsten, and/ or include substantial portions ofsuch rare and expensive elements as niobium and tantalum. In view of thefact that all of the above-mentioned elements may be classified asstrategic materials, and therefore, their availability and cost may besubject to controls other than the law of supply and demand, it is ofimportance that this invention provides a wrought alloy havingmechanical properties 'comparable'with or'superior to any knowncommercially available high temperature wrought alloy without dependingupon excessively'high percentages of cobalt, chromium, nickel,molybdenum or tungsten, and without such elements as niobium, vanadiumand tantalum.

It is a principal object of this invention to provide an alloy havinghigh strength at elevated temperatures which is both chemically andphysically stable underhigh temperature and stress and which is readilyfabricated by the usual methods of forging, rolling .and machining.

It is a further object of this invention to provide wrought articlesmade from a new alloy which will withstand stresses at 1500 F. ofsubstantially the .same magnitude as similar wrought articles made ofknown alloys at 1500 F. without depending upon excessively large amountsof strategic metals.

The alloy forming the subject matter of this invention containsessentially 18 to 22% iron, 18 to 22% cobalt, 26 to 30% nickel, 18 to22% chromium, 6 to 7.5% tungsten, 31.25 to 2.25% molybdenum, 3 to 4.0%titanium and 0.15 to 0.25% carbon.

It is to 'be noted here that certain problems arise when titanium ispresent at one of the alloying elements. Titanium has an exceedinglyhigh rate of reaction with certain of the gases contained .in theatmosphere at elevated temperatures, rendering it next to impossible toalloy more than about 2.5% by weight of titanium in an air meltingprocedure. Further, in view of the fact that relatively small amounts ofoxides, 'hydrides and nitrides of titanium cause asubstantial reductionin the mechanical properties of alloys of this type, .it is preferredthat a vacuum melting process equivalent to that described in mypreviously granted U. '8. Patent 2,564,498-Nisbet, be employed. Thispatent was filed August 26, 1949, given Serial No. 112,558, issuedAugust 14, 1951, and assigned to the assignee of the presentapplication.

A number of heats within the above-stated composition limits wereprepared, heat treated and tested. The tests made included hardnesstests to investigate age-hardening and allied characteristics,stress-rupture tests at elevated temperatures, and conventional tensileand fatigue tests. Representative results of these tests:have'heenselected .for reproduction in order to illustrate certain ofthe characteristics of this alloy.

TABLE I Vickers hardness, 30 kg. load Solution treated at'2,100 F., oilquenched, initial hardness 192 to 194 Aged (hours) 1,400 F. 1,500 F.1,600 F. 1,700 F. 1,800 F.

From the data set forth in Table I, it is apparent that while the alloyexhibits some age-hardening, it is essentially quite stable :at elevatedtemperatures.

TABLE II Tensile properties [Solution treated at 2,150 F., aged at 1,350F.]

Temp, Prop. Ultimate Ductility, Heat Number F. Lim., T. 8., Percent p.s. i. p. s. i. Elong.

Room 95, 500 176, 000 33 1, 200 96, 500 140, 000 18 279 1, 500 78, 00085, 000 1, 600 61, 000 63, 000 29 1, 700 31, 000 31, 000 51 1, 800 50014, 000 110 Room 89,000 148, 000 17 1, 200 86, 000 114, 000 13 299 1,500 ,000 77, 000 4 1, 600 61, 000 62, 000 13 1, 700 32, 000 32, 000 281, 800 13, 500 14, 000 95 Upon inspection of the data disclosed in TableII, it will be noted that the ultimate tensile strength of the alloy isabout 80,000 p. s. i. with an elongation of about 5% when tested at1500" F., for the heat treatment indicated.

TABLE 111 Stress-rupture properties [Solution treated at 2,150 F., agedat 1,350 F.]

Temp., Stress, Life, Ductility, Heat Number F p. 5. Hours PercentElong.inl"

0, 0 2 11500 25,000 100 s '333 133 ii L500 25000 232 19 20,000 524 735,000 60 9 63 16 30,000 13; 299 11500 25,000 252 15 315 6 20,000 552 7577 6 [Solution treated at 2,100 F., aged at 1,400 F.]

2%333 13 i? 165 1,500 301000 62 33'3" iii i3 Upon comparison of thestress-rupture properties set forth in Table 111, it will be noted thatapparently the heat treatment schedule of a solution treatment at 2100F. and aging at 1400 F. produces somewhat superior results over theschedule of solution treatment at 2150 F. and aging at 1350 F. However,in view of all the data, the calculated stress-rupture strength of thisalloy is not less than 18,500 p. s. i. for 1000 hours at 1500 F.

TABLE IV Fatigue properties [Solution treated at 2,150 F., aged at 1,350F.]

1 Test terminated, specimen unbroken.

From the data of Table IV, it is apparent that this alloy has a fatiguestrength of at least 38,000 p. s. i. at 1500 F. for 10 cycles. 5

It should be further noted that these alloys are of austeniticconstituency and have excellent corrosion resistance at elevatedtemperatures.

In the evaluation of an alloy to be used at high temperatures under highstress, the most significant mechanical properties are indicated bystress-rupture and fatigue tests at high temperatures. A second andequally important consideration is the composition of the alloy,particularly with regard to the availability of the various elements ofwhich it is composed. In view of these considerations, a table showingthe compositions, stress-rupture strengths for 1000 hours at 1500 F. andthe fatigue strengths at 10 cycles for several known alloys which are orhave been commercially available having comparable mechanical propertieswill be of assistance for evaluating the alloy of my invention.

TABLE V Composition, Wt. Percent Alloy No. Stress- Fatigue RuptureStrength 0 Cr Ni Co Mo W Fe Nb Ti Al Other 1 Tantalum and/orniobium. 1Fatigue strengths at10 cycles at 1200 F,

While it is not contended that the above alloys are the only known hightemperature Wrought alloys which are or have been commerciallyavailable, these alloys have been selected from those so known as havingthe best mechanical properties and the closest chemical constituency tomy alloy.

It will first be noted that the stress-rupture strengths of these alloysrange from 16,200 p. s. i. to 19,400 p. s. i. for 1000 hours at 1500 F.My alloy has a stress-rupture strength under the same conditions of atleast 18,500 p. s. i. On this basis alone, only alloys Nos. 2, 9 andhave a higher stress-rupture strength.

It will next be noted that the fatigue strengths for these alloys aregiven in pounds per square inch at 10 cycles at 1500 F., except foralloys Nos. 4, 5, 6 and 9, which are only known at 10 cycles at 1200 F.However, based on known behavior of other similar alloys, it may befairly stated that a fatigue strength at 10 cycles at 1200 F. of morethan 60,000 p. s. i. is necessary for alloys of this type to have afatigue strength of approximately 38,000 p. s. i. at 10 at 1500 F.Making this assumption, it is then seen that only alloys Nos. 2, 7 and10 have better fatigue strengths at 10 cycles and 1500 F. than my alloy.

It will then be noted that alloy No. 2 which has only a slightly higherstress-rupture strength and a somewhat higher fatigue strength than myalloy, contains almost twice as much cobalt and tungsten as my alloy,and further, 1.5% niobium, which my alloy lacks altogether. All of theseelements are classified as strategic and are, relatively speaking, quiteexpensive.

In considering alloy No. 7 from the same basis of comparison, it isfirst to be noted that this alloy has a considerably lowerstress-rupture strength, and, for all practical purposes, the samefatigue strength. This alloy, however, contains about two and one-halftimes as much nickel, an element which is becoming more and moredifiicult to obtain, as my alloy and further contains 1% niobium.

Alloy No. 9 has the highest stress-rupture properties of all thesealloys and perhaps the lowest fatigue strength. It contains from 7 to11% more nickel and about 1 to 1.5% more molybdenum than my alloy, butits low fatigue strength renders it quite inferior to my alloy forextended high temperature uses.

Alloy No. 10 has a stress-rupture strength about equal to my alloy, anda better fatigue strength, but it contains about twice as much nickeland about five times as much molybdenum.

From the foregoing, it is apparent that this invention provides awrought alloy having mechanical properties at elevated temperatureswhich are better than or comparable to the best commercially availablewrought alloys and which is further distinguished by the utilization ofmuch lower amounts of critical or strategic metals.

It should be further noted that this alloy contains a much higherpercentage of alloyed titanium than any known wrought high temperaturealloy, and further, it should be noted that titanium is not a criticalor strategic metal.

It is to be further noted, particularly with respect to the titaniumcontent and the preferred vacuum melting technique employed, that thisalloy has very low amounts of inclusions such as metallic oxides,hydrides and nitrides, all of which contributes to its highphysico-chemical properties. These impurities exist in alloys of thistype which have been vacuum melted in percentages of the order of 0.002%oxygen, 0.0001% hydrogen and 0.0005% nitrogen. This is important becauseof the extreme reactivity of titanium at elevated temperatures and theextremely debilitating efiect upon an alloy of appreciable percentagesof titanium oxides, hydrides and nitrides. It is for these reasons thatno air-melted alloy of this type containing more than about 2.5% byweight titanium is 6 known to have a stress-rupture strength at 1500" F.over about 14,000 to 16,000 p. s. i.

It is to be understood that small amounts of elements other than thosespecifically disclosed will inevitably be present in this or any othercommercially produced allay. However, these elements are present only asimpurities and, as such, do not materially alter the composition of thealloy as will later be claimed, nor is it intended that such unspecifiedimpurities will lend any color of patentability to the composition.

Zn view of the foregoing, it is my intention that my invention shall notbe limited in any sense except as defined by he following claims.

it at 1 claim as new and desire to secure by Letters Paent of the UnitedStates is:

1. A high temperature, high strength Wrought alloy consistingessentially of, by weight, 18 to 22% cobalt, 26 to 30% nickel, 18 to 22%chromium, 6 to 7.5% tungsten, 1.25 to 2.25% molybdenum, 3 to 4%titanium, 0.15 to 0.25 carbon, and the balance being substantially alliron with the amounts of the other constituents being so selected thatthe iron content lies within the range 18 to 22%.

2. A high temperature, high strength wrought alloy consistingessentially of, by weight, 18 to 22% cobalt, 26 to 30% nickel, 18 to 22%chromium, 6 to 7.5% tungsten, 1.25 to 2.25 molybdenum, 3 to 4% titanium,0.15 to 0.25% carbon, the balance being substantially all iron with theamounts of the other constituents being so selected that the ironcontent lies within the range 18 to 22%, and said alloy containing lessthan 0.006%, by weight, of free and combined oxygen, hydrogen, andnitrogen.

3. A high temperature, high strength wrought alloy consistingessentially of, by Weight, about 18% cobalt, 29% nickel, 20.5% chromium,7.4% tungsten, 2% molybdenum, 3.5 titanium, 0.20% carbon, and thebalance being substantially all iron.

4. A high temperature, high strength wrought alloy consistingessentially of, by weight, about 18 cobalt, 29% nickel, 20.5% chromium,7.4% tungsten, 2% molybdenum, 3.5% titanium, 0.20% carbon, the balancebeing substantially all iron, and said alloy containing less than0.006%, by weight, free and combined oxygen, hy drogen, and nitrogen.

5. A wrought article having a stress-rupture life of at least 1000 hoursat 1500 F. under a load of 18,500 p. s. i. and a fatigue strength at1500 F. of at least 38,000 p. s. i. for 10 cycles, said article beingcomposed of an alloy consisting essentially of, by weight, 18 to 22%cobalt, 26 to 30% nickel, 18 to 22% chromium, 6 to 7.5% tungsten, 1.25to 2.25 molybdenum, 3 to 4% titanium, 0.15 to 0.25% carbon, the balancebeing substantially all iron with the amounts of the other constituentsbeing so selected that the iron content lies Within the range 18 to 22%,and said article containing less than 0.006%, by weight, free andcombined oxygen, hydrogen, and nitrogen.

6. A wrought article having a stress-rupture life of at least 1000 hoursat 1500 F. under a load of 18,500 p. s. i. and a fatigue strength at1500 F. of at least 38,000 p. s. i. for 10 cycles, said article beingcomposed of an alloy consisting essentially of, by weight, about 18%cobalt, 29% nickel, 20.5% chromium, 7.4% tungsten, 2% molybdenum, 3.5titanium, 0.20% carbon, the balance being substantially all iron, andsaid article containing less than 0.006%, by weight, free and combinedKygen, hydrogen, and nitrogen.

References Cited in the file of this patent UNITED STATES PATENTS2,246,078 Rohn June 17, 1941 2,397,034 Mohling Mar. 19, 1946 2,475,642Scott July 12, 1949 2,513,469 Franks et a1. July 4, 1950

1. A HIGH TEMPERATURE, HIGH STRENGTH WROUGHT ALLOY CONSISTINGESSENTIALLY OF, BY WEIGHT, 18 TO 22% COBALT, 26 TO 30% NICKEL, 18 TO 22%CHROMIUM, 6 TO 7.5% TUNGSTEN, 1.25 TO 2.25% MOLYBDENUM, 3 TO 4%TITANIUM, 0.15 TO 0.25% CARBON, AND THE BALANCE BEING SUBSTANTIALLY ALLIRON WITH THE AMOUNTS OF THE OTHER CONSTITUENTS BEING SO SELECTED THATTHE IRON CONTENT LIES WITHIN THE RANGE 18 TO 22%.